The present invention relates to continuous ink jet printing and, more particularly, to preventing ink from dripping following a startup sequence for a continuous ink jet printhead, when the eyelid opens to the print position.
Ink jet printing systems are known in which a printhead defines one or more rows of orifices which receive an electrically conductive recording fluid from a pressurized fluid supply manifold and eject the fluid in rows of parallel streams. Printers using such printheads accomplish graphic reproduction by selectively charging and deflecting the drops in each of the streams and depositing at least some of the drops on a print receiving medium, while others of the drops strike a drop catcher device.
In normal operation of the printhead, the charging electrodes deflect most of the ink drops, causing them to strike the catcher face. The ink then flows down the catcher face and enters the catcher throat. Vacuum then draws the ink through the catcher outlet port back to the ink reservoir.
Ink removal means have been applied in the ink jet printing field, such as by the use of screens in catchers, for example, in U.S. Pat. No. 5,105,205, and U.S. Pat. No. 5,469,202. In the prior art, screens have been used to fill the front of the catcher throat or fluid channel. A portion of those folded screens lies on the bottom surface of the catcher flow channel, and bridges the gap to the upper surface of the fluid channel. In that position, all the ink must flow through the pores of the screen to enter the catcher flow channel. The flow of the ink through the pores of the screen produces a significant pressure drop through the screen. The pressure drop through the screen, in combination with the unobstructed flow channel behind the screen, serves the stated purpose in the prior art of providing uniform ink removal across the width of the flow channel.
During the automatic startup sequence of a continuous ink jet printhead, the fluid pressure to the ink jets can be anywhere from a low pressure where ink xe2x80x9cweepsxe2x80x9d from the droplet generator to the final operating pressure. By way of example, for some printheads, the startup sequence can include states where ink weeps at low pressure from the droplet generator, to help redissolve ink on the exterior of the orifice plate and on the charging electrodes; states where ink is jetted out of the droplet generator orifices at a pressure lower than the operating pressure to allow condensate cleaning and drying of the charge plate; and states where the ink pressure is at the operating pressure, prior to turning on the drop charging to deflect the droplets onto the catcher face.
During the startup sequence, eyelid means are used to seal against the bottom of the catcher. The eyelid sealing means not only seal against the catcher, but are also designed to divert ink that is jetting from the drop generator into the catcher throat. It has been determined that this process of diverting ink flow into the catcher throat by means of the eyelid has much higher fluid flow energy losses than the process of having the ink drops strike the catcher face and then flow into the catcher throat. As a result, a catcher ink return geometry that can effectively remove ink from the printhead when the drops are deflected into catch may have too much restriction to remove ink that is diverted into the catcher throat by the eyelid. In particular, it has been noted that while screens can be employed in the catcher throat for certain short array printhead configurations, that they produce excessive flow impedance for proper operation of longer array printheads having higher flow rates. For such systems, a fluid return geometry as described in U.S. Pat. No. 6,187,212 has been found to be more effective.
The flow rate of ink in ink jet printheads developed since the development of the ""212 flow geometry have increased from 750 ml/min to 1300 ml/min. Small changes in the flow geometry in keeping with the teachings of the ""212 patent have allowed the flow return channel to handle the increased flow rates. However, it has been found that while the flow return channels can handle the ink both during startup and when in catch, during the transition into catch enough ink remains at the front of the flow return channel in contact with the eyelid seal, to drip and to bridge the opening between the eyelid seal and the catch plate edge. Even when this ink bridge is removed manually, closing the eyelid and then reopening it would cause ink to pull out from the catcher throat.
It would be desirable, therefore, to be able to remove excess fluid that can remain on the eyelid seal at or near the catch plate edge after the ready startup cycle of a printhead, particularly for high flow rate printheads, without introducing a pressure drop at the entrance to the catcher fluid channel.
An appropriately placed wicking medium, in combination with surface tension, solves the problem of excess fluid remaining on the eyelid seal. During the ready startup sequence of certain printheads, particularly printheads having high flow rates, the transition of jetting into the catch plate and the eyelid seal to the application of a charge potential for deflecting the droplets onto the catcher surface at the eyelid seal, can cause ink to be left on the eyelid seal, which ink drips after the eyelid is opened to the print position. In accordance with the present invention, a stainless steel mesh screen is placed on the catch plate of the printhead assembly to eliminate the ink on the seal, and to keep the ink away from the catch plate edge to eliminate ink pullout.
In accordance with one aspect of the present invention, an inkjet printer has a catcher and eyelid seal for sealing against the catcher during startup and shutdown. A wicking means is provided for removing excess ink from a fluid channel of the catcher having an associated catcher plate, in an area of the eyelid seal. The wicking means is positioned in an area at a bottom surface of the fluid channel, without bridging a height of the fluid channel, above the catcher plate, and in close proximity to the eyelid seal, while maintaining a consistent pressure at an entrance to the fluid channel.